Process of producing aluminum oxide substantially free from silica



Patented June 10, 1 947 I PROCESS OF PRODUCING ALUMINUM OXIDE SUBSTANTIALLY FREE FROM SILICA Frank J. Anderson and Duncan R. Williams,

Laramie, Wyo., assignors to Monolith Portland Midwest Company, Lar

tion oflNevada amie, Wyo., a corpora- Application September 20, 1944, Serial No. 554,974

2 Claims; (Cl. 23-143) Our invention relates to a process by which various earths containing kaolin, marl, or the like with various impurities, such as silica, may be utilized economically in the production of aluminum oxide. 5 fine ourselves to a discussion of a plant using A great proportion of the crust of the earth kaolin and marl, since both are abundant and consists of materials which are largely compounds cheap and in combination work very well in our of aluminum, calcium, and silicon, and which are process. commonly designated y a es that describe Various mixtures of minerals which contain their physical rather than their chemical char about the desired proportions of aluminum, calacteristics. Large areas of what is commonly eium, and silicon may be used, and it is an essencalled waste land contain immense deposits of 13 ,1 d t g of our process t t an exact Such materials. and S e the landv itself has Very ture is not necessary to the success of our process little Value, the 00st of Such materials s a e for reasons that will be made evident hereinthe cost of digging and transporting such mate- 15 after. It, i ,1 n advantage of our process that rials. These materials contain not only the valif th r at rial ar mix d in v ry roughly uable aluminum, calcium, and silicon in combithe proper proportions, there is very ttl waste nation with each other and other elements, but materia} produced, and commercially salable they contain othermaterials, for example, iron productsare pmduced salts, sodium and potassium compounds, and or- Many materials may be used as t prime ganic matter, and none of the components can be Source f aluminum and the kaolins are cheap separated without expensive processing. It is an and abundant source of such materiaL In object of our invention to provide a process-by d ribing t operation f a typical plant, we which such inexpensive raw materials may be shall assume that such a kaolin is used, an analprocessed to produce an aluminum oxide of sufysis of this kaolin which is more or less typical ficient purity to make it available for the probeing: duction of aluminum.

Among related materials which may be treated Per cent by our process are nearly all naturally occurring S10 45.8 minerals and mixtures of minerals containing F6203 0.4 silicon, aluminum, and calcium, such as the feld- A1 0 39,5 spars, particularly anorthosite, shales, clays, etc., L 14.3 and impure bauxite.

Our process will be better understood by in- Under loss we include materials actually found spection of a diagram illustrative of an assembly in the kaolin which are of no value in, and which of apparatus such as used in a plant for prachave no effect on, the process. The loss in the ticing our invention. In this diagram we have kaolin is largely chemically combined Water. not attempted to illustrate any details of the To provide approximately the proper mix with apparatus actually used by us, as each piece of the kaolin for producing the desired products, apparatus so used is well known in the art and w may use 1imestone or any material high in u d be p d a d d y p at d y y calcium. In the plant to be hereinafter deman Skilled in the art after having read this scribed, we would use amarl having the following specification. analysis The four principal elements of the products produced by the process are aluminum, calcium, 5 Per cent sodium, and silicon. Silicon is found in minerals, SiOz 12.5 not only as silica (S102), but also in the form F6203 2.3 of the aluminum and other silicates. Aluminum A1203 2.3 may be present as aluminum oxide and hydrox- CaO 45.0 ide as well as aluminum silicate. Calcium may Loss 37.9 appear as carbonate, oxide, or hydroxide. In general, almost any combination of raw material n S ating analyses herein, for convenience i that contains the necessary amounts of alumichemical calculation, we list each of the metals, num and calcium and which also contains silica including silicon, as if it were present in the or silicates can be used. We know that kaolin form of its oxide. As a matter of fact, it may .and marl make an excellent combination, but

other combinations, such as the feldspars (for example, anorthosite) and limestone, can be used. In the following description, we shall conbe in the form of a hydroxide, carbonate, or other salt, or two metals may be united to form a double salt. The specific chemistry will be later discussed.

The loss in the marl is largely CO1, since the calcium is present for the most part as a carbonate.

The above materials, kaolin and marl, are selected because large deposits of such materials are available, and we have definite data, which will be hereinafter referred to, as to the operation of a plant using these materials. Any competent chemist skilled in the art with the information given him herein could modify the process to suit other raw materials.

The raw materials, kaolin and marl, containing various impurities as noted, are mined usually by power shovels and, without attempt at more than a very crude classification, are broken up into convenient size for handling and dumped into storage bins I and 2. Fluorspar may be provided in the bin la. These materials are then passed in proper proportions to a slurry mill 3, where they are mixed with barren solution and ground. Sodium compounds are necessary to the operation of the process and are added in solution in what we term barren solution. We shall later decribe the source of this solution. In the operation of our plant producing 60 tons of alumina during each twenty-four hours, the following amounts of material will be passed to the slurry mill 3 during that period:

One of the advantages of our process is that the materials are mixed in a slurry, the composition of which can be easily determined and controlled. Such a. slurry is also easil transported by means of pumps and piping.

The sodium in the barren solution is derived initially from soda ash supplied to the process, but during the operation of the process there is very little loss of sodium, and that recovered is reused. The raw slurry, however, must contain enough sodium to allow substantially all the aluminum present to combine with sodium to form sodium aluminate, allowing for small sodium losses. Operating on raw materials containing sodium, which is quite practical, the amount of soda supplied by the barren solution may be reduced, and in some cases where considerable sodium is present in the raw material, considerable sodium carbonate may be produced as a by-Drodnot of the process.

We find it desirable to also add fluorspar to the slurry in an amount equal to from /2 to 3% and and thoroughly mixed in a slurry 'mixer 4 and passed to a first drying kiln 5 and from the drying kiln 5 to a sintering kiln 6. The kiln 5 will receive during each twenty-four hours in the slurry:

. Tons SiOz 130.10 F8203 12.59 A1203 76.56 CaO 239.47 NazO 52.56 Loss 217.29 H2O 560.66

Although the above analyses are given in terms of the oxides of the various elements for convenience in chemical calculation, it is to be understood that these oxides may not be present as such, as, for example, the aluminum and silicon may be combined as an aluminum silicate, and the calcium is present as a carbonate.

The mill 3 may be of any form suitable for grinding the slurry. In our plant the kilns 5 and B are rotary kilns internally fired, such as are used in cement manufacture. In the drying kiln 5 the uncombined water of the slurry is largely evaporated. An operator skilled in the operation of such kilns can so regulate the rate of feed, rotation, and firing that the material is delivered from the kiln 5 to the kiln 6 in a'substantially dry state, without the adhesion of the materials to the walls of the kiln and without the formation of rings in the kiln. If rings so form, they will be in a very accessible location near the outlet end of the kiln. The operation of the kiln 6 must be carefully regulated as, while in the interest of economy it is desirable to use all the aluminum present to form sodium aluminate, if the material is held in the kiln for too long a period or the temperature is carried too high, a portion of the aluminum is converted into other products, such as sodium-aluminum silicate or aluminum silicate. At the proper temperature a sinter is formed which is largely free from glass and which is not slagged or melted into large masses. A typical analysis of this sinter is:

Per cent SiOz 25.47 F6203 2.46 A1203 14.97 CaO 46.84 NazO 10.28

and each twenty-four hours we produce in the sinter:

Tons

s10: 130.10 F8203 12.59 A1203 76.56 CaO 2 39.47 NazO 52.56

In these kilns 5 and 6 the aluminum silicate is substantially entirely converted into sodium aluminate, which is water-soluble, the silica combining with lime to form dicalcium silicate, which is water-insoluble. The 560.66 tons of water contained in the slurry is, of course, driven off largely in the first kiln 5. A hydrocarbon oil or gas or coal or coke is preferably used to fire these kilns, and the hot gases from the kiln Ii, which are largely CO2, are passed to a purifier I in which the products are cooled to condense steam and scrubbed to remove tar and solid matter, the CO2 being then passed to a carbonator 8. By using two kilns and taking out the water from the slurry in the first kiln 5, the products of combustion from the second kiln do not contain the steam due to vaporization of the water of the slurry, and the purifier I does not have to handle as much steam as it would if the water of the slurry was passed as steam intothe purifier I.

The sinter from the kiln 6 is passed to a sinter mill 9, where the sinter is ground to pass through a (SO-mesh screen and wet with barren solution, which each twenty-four hours supplies the following amounts of material:

Tons

S102 0.02 A1203 1.51 NazO i 16.76 H2O -1 233.07

It is important that enough sodium be added at this point to enable a stable sodium aluminate solution to be produced. In fact, enough sodium should be added to produce a mol ratio of not less than and it can be as much as 1.9. Unless there is excess sodium in the solution which is then formed, alumina will precipitate from the solution and be carried into the cement, as will be understood after the remainder of the process has been described. It is an advantage of our process that the Portland cement produced thereby is very low in aluminum content, which is a very desirable characteristic. The sodium can, of course, be derived from the barren solution, but, since it is often necessary to add sodium to make up for losses. the soda ash or other soda compound necessary for this purpose can be conveniently added at this point.

We have found, however, that if the sinter from the kiln 6 is simply mixed with water or even a solution in which an excess of sodium is present, the sinter tends to gel or stick together, which interferes with the leaching and subsequent handling of this sinter. We therefore add to the material in the leaching tank from 0.2 to 2.0% of the weight of the sinter of a suitable sugar, such as molasses or corn syrup.

Soda ash or other soda compound during each twenty-four hours supplies the following material to the sinter mill 9:

Tons NaaO 4.31 Loss 3.13

The slurry from the sinter mill 9 passes to a leacher III. This is shown in the diagram as a Per cent SlOz 32.60 F6203 1.80 A1203 3.20 CaO 60.50 Naro 1.10 Loss 0.80

and there should be delivered to the cement slurry tank I I each twenty-four hours:

Tons S102 128.93 F8203 12.59 A1201 7.33 CaO 239.47 NazO 4.15 Loss 3.13 H2O 233.07

\ This residue is too high in silica and too low in calcium to make good Portland cement, the silica and calcium being present as dicalcium silicate, and tricalcium silicate being desired in cement manufacture. We therefore add 462.06 tons of marl to the cement slurry tank II during each twenty-four hours. This marl is delivered from a marl bin I2 through a marl mill I3 to the cement slurry tank I I. The marl used by us has an analysis as follows:

Per cent SiOz 12.5 FezOa 2.3 A1203 2.3 CaO 45.0 Loss 37.9

largely sodium aluminate carrying some silica. or

silicates. The sugar is still carried in solution. In each twenty-four hours the following amounts of material should pass from the leacher I0 to an autoclave I 6 Tons Slog 1.19 A1203 70.74 NazO 69.48 H2O 1 962.07

It will be noted that the percentage of silica carried in this solution is low, but it is still too high for our purpose. We therefore subject the extracted solution, to which slaked lime has been added, to digestion in the autoclave I6. We need enough Ca(OH)z per twenty-four hours to provide 5.13 cons of CaO for this purpose. We find that it is desirable that the lime be freshly burned and freshly slaked. The autoclave is supplied with steam at 100 pounds pressure and maintained at that pressure and at the temperature of steam at that pressure for a period of at least one hour. There is an excess of lime in the form of Ca(OH)z in the autoclave. It is only slightly soluble, and the remainder is in suspension. The principal result of the autoclave operation is to convert the silica into insoluble compounds, notably calcium silicate. It is of advantage to add to the material in the autoclave IS an amount of bentonite about equal to the amount of lime used, this bentonite acting to adsorb and collect fine particles of silica, which are retained in the bentonite when it is separated in a separator ll. After autoclavlng, the solution is passed to the separator H, where insoluble residues are extracted and then returned to the slurry mill 3. In the separator H, 4.73 tons of wash water are adding during each twenty-four hours, and the desilicated solution is passed to the carbonator 8. We pass to the carbonator each twenty-four hours:

The soda and alumina are present in the solution passing to the carbonator 8 as a sodium aluminate carried in an alkali solution containing a sugar.

In the carbonator 8 carbon dioxide gas derived from the second sintering kiln 6 and purified in the purifier is bubbled through the solution and thoroughly mixed therewith. We so use 44.48 tons of CO2 each twenty-four hours. The carbonation converts the sodium aluminate into water-soluble sodium carbonate and water-insoluble aluminum hydrate. We have found that if the carbonation is carried too far, silica is precipitated from the solution, and we prefer to carbonate only from 60 to 90% of the aluminum out of the solution. This can be regulated by observing the hydrogen ion concentration in the carbonator and stopping the carbonation when the pH is reduced to about 13. From the carbonator 8 the solution carrying suspended solids is passed to a classifier I 8. In the classifier the barren solution is separated from the solid residue, and the solid residue is separated into fine particles or seed, which is sent back to the solution flowing to the carbonator, and coarse particles, which are sent to a washer 2|. These coarse particles still carry water, and there are substances in solution in the water which we do not desire in our final product. It is therefore necessary to thoroughly wash the solid residue of the classifier l8, which is done with distilled water in the washer 2|, this water being condensate from an evaporator 20. The wash water with dissolved impurities is sent back to the classifier l8 from the washer 2|. In plant operation we use 277.2 tons of this wash water every twenty-four hours.

We have found that, in spite of the steps previously taken to eliminate silica, some silica remains in the material passing from the classifier l8 to the washer 21. We find that at times as much as 14.7 pounds of silica per ton of aluminum hydroxide will be found in the material passing from the classifier l8 to the washer 2|. If this proportion of silica were left in the aluminum hydroxide passing to the calcining kiln IS, the proportion of silica in the desired aluminum oxide would be too high to permit the oxide to be used tons of aluminum fluoride must be used each twenty-four hours. The aluminum hydroxide and aluminum fluoride are then passed to the calcining kiln l9, in which the fluorine combines with the silicon to produce silicon tetrafluoride,

which volatilizes in the kiln l9. In this kiln the hydrate is changed to aluminum oxide, and about 60 tons of such oxide are produced during each twenty-four hours. If the process has been operated as above described, only a trace of silica is present in the oxide. A typical analysis of this product may show the following impurities:

Per cent SiOz .01 F6203 .015 TiOz .010 K20 .020 NazO .300 CaO .015 MgO .008

These impurities are, of course, carried into the process by the raw materials. Since we do not attempt to purify the raw materials used, and the amount of these impurities is within commercial limits, we have not attempted to find their source. An aluminum oxide containing such small quantities of impurities is a valuable product, as it is a metallurgical alumina which is in great demand.

The solution from the classifier is evaporated in the evaporator 20, 409 tons of water being evaporated each twenty-four hours. This evaporation is desirable to increase the percentage of NazO in the barren solution to a point that will enable slurry of the proper consistency to be formed in the mills 3 and 9. The sodium of the barren solution, although given in the analyses as NazO, as it leaves the evaporator is in the form of carbonate and aluminate, but it loses its CO2 in the kilns 5 and B.

It will be noted that the plant operates at low cost and may utilize kaolin, marl, shale, or other minerals without preliminary purification, and that substantially all the low cost raw material used is converted either to aluminum oxide or Portland cement. There is little or none of the waste removal problem so commonly found in plants of this general character. It will further be noted that, although soda ash or other soda compound must be initially supplied to provide the sodium needed for the operation of the process, this sodium is recovered and reused, and the loss in sodium is small, and therefore only those small amounts must be supplied which are necessary to provide for losses.

The chemistry of this process may be described briefiy as follows:

Th raw materials used in the particular example of the process discussed, for example, marl and kaolin, contain aluminum, silicon, and calcium, but are relatively free of potassium and sodium. By adding barren solution containing the necessary sodium to the slurry in the slurry 9 mixer l, a slurry is produced which when burned in the kilns and 6 will cause the alkali, which in the example given above is sodium carbonate, to unite with substantially all the aluminum to form water-soluble sodium aluminate, which is leached out of the insoluble residue in the leacher Hi. This residue will not make a satisfactory cement, but when enriched with marl or other raw material high in calcium produces a slurry which when burned in the kiln l4 and ground in the mill l5 does produce av very good cement which is notably low in aluminum. v a

The sodium aluminate solution from the leacher I0 is not a good raw material for the production of aluminum oxide, since it carries considerable amounts of silica. By adding lime and bentonite to this solution and treating it in the autoclave Hi, this silica become calciumsilicate which may b separated from the solution and sent back to the slurry mill 3 or the slurry tank 4.

The solution of sodium aluminate from the autoclave is then passed to the carbonator 8, where the greater portion of it is converted to aluminum hydrate and sodium carbonate. In the classifier l8, fine particles of aluminum hydrate are separated from the solution and sent back to the carbonator to act as nuclei for aluminum hydrate formation. The residue is then washed, and aluminum fluoride is added, which acts to carry off all but a small trace of silica. The sodium carbonate solution is then evaporated in the evaporator 20 to produce a barrenfsolution of sodium carbonate and sodium aluminate ordinarily containing enough sodium to supply the needs of the process.

The above process has the advantage that very low cost raw materials ar used, and very little of even these materials is wasted, substantially all of them being converted into valuable aluminum oxide and Portland cement. The process is practiced in apparatus such as used in the manufacture of Portland cement, which is easily operated by unskilled labor without onerous supervision. By our process kaolin and marl may be used to produce aluminum oxide suitable for use in the production of the metal aluminum, which is now so greatly in demand, and as to which there is a shortage due to lack of high grade bauxite, By using the process as described to produce aluminum oxide and Portland cement simultaneously, both can be produced at low cost.

The process may not only be used on raw materials such as kaolin and marl or feldspar and shale, but in fact on many other materials containing calcium, aluminum, and silicon. If the raw materials contain sodium or potassium in considerable amounts, it may be economical to produce potash or soda as a by-product of the process by evaporation of excess barren solution. The process can be used to produce the oxides of beryllium, titanium, and vanadium, as well as aluminum oxide, as these metals act just as aluminum does in the process, or it may be used on a raw material containing two or more of these metals and producing a mixed oxide,that is, a mixture of aluminum and beryllium oxide, for example. The separation of such oxides into their component parts is well understood.

The process is characterized by certain claims appended hereto and written in dependent form for convenience in examination. Permission will be requested to rewrite these claims in non-dependent form when the examination is completed.

This application is a, continuation-in-part of our application Serial No. 451,802, filed July 21,

, compound, and an alkali-metal carbonate with water, said carbonate being supplied insufficient amount to produce a mol ratio of not less than NazOtAlzQatZL511 in the slurry; heating the slurry for a time and at a temperature sufficient to drive off the water and to form a sinter containing silicon, calcium, aluminum, and the al--.

kali-metal; grinding the sinter to permit ready leaching; leaching said sinter with water containing an alkali-metal carbonate for a suflicient period to take into solution all water-soluble material in said ground sinter; subjecting the solution so formed to heat and pressure in the presence of bentonite and lime to precipitate from the solution a large proportion of the silica initially present in said solution; removing the precipitated silica from said solution; carbonating the solution obtained by said leaching; separating from said carbonated solution water-insoluble solids produced by said carbonation; adding to said solids sufficient aluminum fluoride to combine with any residual silica in said solids to form silicon tetrafluoride; and calcining the mixture of said solids and silicon tetrafluoride at sufliciently high temperature and for a sufilcient. time to form and vaporize said silicon tetra. fluoride which is allowed to escape from the solids, and simultaneously to convert aluminum hydroxide to aluminum oxide.

2. A process of treating naturally occurring earths, such as kaolin, marl, and other earths containing silicon and aluminum, for the purpose of producing aluminum oxide, which comprises: forming a slurry by mixing said earth, a calcium compound, and an alkali-metal carbonate with water, said carbonate being suppliedin suflicient amount to produce a mol ratio of not less than NazO2A12O322L521 in the slurry; heating the slurry for a time and at a temperature sufficient to drive off the water and to form a sinter containing silicon, calcium, aluminum, and the al- 7 kali-metal; grinding the sinter to permit ready leaching; leaching said sinter with water con-- taining an alkali-metal carbonate and sugar for a sufficient period to take into solution all watersoluble material in said ground sinter; subjecting (iii the solution so formed to heat and pressure in the presence of bentonite and lime to precipitate from the solution a large proportion of the silica initially present in said solution; removing the precipitated silica from said solution; carbonating the solution obtained by said leaching; separating from said carbonated solution water-insoluble solids produced by said carbonation; adding to said solids sutficient aluminum fluoride to combine with any residual silica in said solids to form silicon tetrafluoride; and calcining the mixture of said solids and silicontetrafiuoride at sufliciently high temperature and for a sufficient time to form and vaporize said silicon tetrafluoride which is allowed to escape from the solids, and simultaneously to convert aluminum hydroxide to aluminum oxide.

FRANK J. ANDERSON. DUNCAN R. WILLIAMS.

(References on following page) The following references are of record in the REFERENCES CITED Number Name Date Pontoppidan Oct. 28, 1919 Childs Aug. 20, 19121 Chappell Nov. 25, 1913 Spence et al Oct. 19, 1915 Coles May 19, 1942 Stohr June 1, 1937 Cowles Nov. 11, 1924 Sherwin July 4, 1922 Seailles Sept. 28, 1943 Number 10 Number Name Date Seailles July 8, 1941 Folger Dec. 20, 1938 Stohr Oct. 30, 1934 Scheidt Aug. 28, 1934 Cowles July 6, 1926 Howard May 4, 1915 FOREIGN PATENTS Country Date Switzerland Mar. 1, 1930 OTHER REFERENCES Kammermeyer et al., The Extraction of Alu- Engineers, vol. 36, pp. 683-700 (1940). 

